In the near future, the optical free-space communication between satellites, as well as between satellites and ground stations, will constitute an important supplement to the existing microwave technology, which also saves weight on board the satellite. So-called optical terminals consist of one or several telescopes, which limit the angular range of the field of view of an optical receiver in the direction toward a counter-station, and also provide a directional radiation of the signals to be transmitted. Several movable mirrors are furthermore provided, by means of which the alignment of the transmitting and receiving directions is performed. Besides the direct detection of the optical output of the transmitter of the counter-station constituting the transmission process, the coherent superimposition of the received light with the light of the same frequency of a local oscillator laser plays an important role since, besides great sensitivity to the signal to be detected, the insensitivity to interference by background radiation is important.
The diode lasers, which have reached a high degree of development because of their extensive application in fiber-optical communications, also represent an alternative, at least for simple systems operating with intensity modulation, and in addition also save space and weight. But in spite of their operation on only a single optical frequency, which has also been achieved here, they are generally not yet suitable for coherent transmission processes, except for complicated structures with large, additionally coupled-in resonators. One reason is the still too large spectral width of this radiated optical frequency. Although fiber-optical coherent transmission systems do operate with customary commercial diode lasers, because of the line guided transmission, detection takes place with a relatively large optical output.
But the background radiation, which is interferingly present in free-space transmissions, as well as the mostly very low power of the received signals, require an optical bandwidth of the unmodulated signal which is considerably narrower than the modulation bandwidth. This is a requirement which, together with small size and low weight, can be best met by diode laser-pumped solid-state lasers. Existing attempts to integrate the laser systems required for operation into a terminal for optical free- space communications have been described by Carlson et al., as well as by Marshalek et al. (R. T. Carlson et al., "Monolithic Glass Block Lasercom Terminal: Hardware Proof of Concept and Test Results", SPIE, vol. 2381, Free-Space Laser Communications Technologies VlI, Feb. 7-8, 1995, San Jose, Calif., pp. 90 to 102; R. G. Marshalek et al., "Lightweight, High Data-Rate Laser Communications Terminal or Low Earth Orbit Satellite Constellations", SPIE vol. 2381, Free-Space Laser Communications Technologies VlI, Feb. 7-8, 1995, San Jose, Calif., pp. 72-82).
Both groups of authors describe laser systems which are mechanically coupled to the optical devices of a terminal and which guide their light emissions into the optical device by means of collimated beams. However, diode lasers in accordance with the state of the art have been used here.
Diode laser-pumped solid-state lasers have a larger volume and lower efficiency and therefore generate a larger amount of waste heat than comparable diode lasers. The increased amount of heat produced in the vicinity of the optical system has been shown to be a risk for the undisturbed operation of the optical device.
The insufficient modulation capability of diode laser- pumped solid-state lasers represents a further problem. In contrast to diode lasers, the medium generating the optical amplification remains in an excited state for a relatively long time after the supply of pump energy. Furthermore, the resonator of such lasers is considerably larger than that of diode lasers. As a result, cut-off frequencies of approximately 100 kHz are typical for amplitude modulation, for example. The external modulation required for this is also quite difficult to perform, since a high optical power must be manipulated, which requires the use of electro-optical modulators which have low cut-off frequencies.
External modulation of laser light can be performed at high cut-off frequencies in modulators in which the light is conducted in a waveguide, which permits a small mutual distance between the electrodes that provide the modulating voltage, and therefore permits a lower modulation voltage. Since, because of the strong increase of the optical intensity caused by the narrow cross section of the optical waveguide, this method only permits low optical output, the modulated optical signal must be post- amplified. Attempts to do this consist in the application of processes and devices which, in the meantime, have proven themselves in fiber-guided optical communications, for example by means of the post-amplification of the modulated optical signal with a fiber amplifier doped with erbium (T. Araki, M. Yajima. S. Nakamori, Y. Hisada, "Laser Transmitter Systems for High Data-Rate Optical Inter-Orbit Communications", SPIE vol. 2381, Free-Space Laser Communications Technologies VII, Feb. 7-8, 1995, San Jose, Calif., pp. 264-272).
Besides diode laser-pumped solid-state lasers, appropriate traveling wave amplifiers are also used, wherewith, especially for the post-amplification of light, devices operating with lasers from the same technology are available, in particular for diode laser-pumped neodymium-YAG solid-state lasers, which are very useful for optical free-space communications because of their narrow spectral width. The light to be amplified is conducted into an amplifying crystal, in which the photons of the light beam, with a defined probability, meet atoms which are in an optically excited state, which is comparatively stable over time because of the special properties of the material. The relative stability of this state is interrupted by a photon having the same energy as the difference between the excited state and the lower laser level of the atom, wherewith the respective atom releases an additional photon with the same wavelength (i.e. the same energy) and phase.
The excited state of the atoms is caused by so-called pump light, which generally has a shorter wavelength than the light to be amplified and puts the atoms in an excited state corresponding to the energy of its photons, from which the latter spontaneously change into a relatively stable state, whose energy difference with the non-excited lower laser level corresponds to the energy of the photons of the light to be amplified. A high amplification of the light is achieved if, during the passage through the amplifying medium, the photons of the light to be amplified meet many excited atoms. The volume density of excited atoms therefore must be very high. However, since a certain portion of the excited atoms per unit of time spontaneously transits into the lower laser level because of a finite average lifetime of the excited state, and the photon emitted in the process is lost for the amplification of the light, it is necessary to continuously pump light with a high intensity into the medium, even when there is a lack of light to be amplified, in order to maintain the high volume density of excited atoms. At low input intensity such devices provide high amplification factors but, their efficiency is extremely low. On the other hand low amplification factors are observed when the light to be amplified already has a high intensity, i.e. if a large average rate of photons passes through the amplifying medium and the density of excited atoms is reduced because of a high rate of stimulated emissions of additional photons.
After a short average time each atom excited by the pump light transits into the lower laser level induced by a photon of the light to be amplified. With a comparatively long average lifetime of the excited atoms, there is a comparatively low probability of a spontaneous, and therefore useless, transit to the lower laser level, because of which the efficiency at high intensity and therefore low amplification is high.
In order to achieve a high amplification, along with a simultaneously high rate of stimulated transits into the lower laser level, it is necessary, despite the low density of excited atoms in the amplifying medium, to assure a large average number of additional photons generated by stimulated transits of excited atoms into the lower laser level. This is mostly achieved in that the light to be amplified is guided over as many paths as possible through the zone of an amplifying medium irradiated with pump light. With a respectively constant volume density of excited atoms, for each photon of the light to be amplified the probability to generate additional, stimulatedly emitted photons is multiplied by the number of paths through the gain medium.
It is therefore possible to generate a comparatively high amplification factor in spite of low pump power. However, the devices in accordance with the state of the art are constructed of several elements requiring a lot of space and mass, which therefore only poorly satisfy space travel-specific requirements. Special developments also contain the risk of insufficient mechanical stabilities (T. J. Kane, E. A. P. Cheng, B. Nguyen, "Diode-Pumped ND:YAG Amplifier with 52 dB Gain", SPIE vol. 2381, Free-Space Laser Communications Technologies VII, Feb. 7-8, 1995, San Jose, Calif., pp. 273-284; T. E. Olson, T. J. Kane, W. M. Grossmann, H. Plaessmann, "Multiple Diode-Pumped ND:YAG Optical Amplifiers at 1.96 .mu.m and 1.32 .mu.m", Optical Letters, vol. 6, No. 5, May 1994, pp. 605-608).
An additional problem for space travel applications consists in that the diode lasers used for generating the pump light also have a limited lifetime. Accordingly it is necessary to keep several diode lasers in reserve for every diode laser-pumped solid-state laser and each diode laser-pumped optical amplifier in order to be able to replace broken-down ones.
But diode lasers provided in redundancy require optical devices which permit switching between the light beams emitted by the individual laser diodes.